Method for manufacturing a broadband cholesteric polarizer

ABSTRACT

The Application describes a method and devices for manufacturing a broadband cholesteric polarizer having a well-defined bandwidth and edge position. To this end, a liquid-crystalline cholesterically ordered layer comprising reactive chiral monomers and reactive nematogenic monomers having a different reactivity is polymerized by exposure to radiation. The invention is characterized in that the intensity of the radiation is increased substantially, preferably by a factor of 10 or more, when a desired edge position of the band is reached. As a result, the bandwidth and edge position of the reflection band reached at that moment are frozen as it were. A monochromatic sensor can be used to determine whether said edge position has been reached, the wavelength used by the sensor corresponding to the wavelength of the desired edge position of the band. Broadband, cholesteric polarizers having a well-defined bandwidth and edge position can very advantageously be used in a display.

[0001] The invention relates to a method of manufacturing a broadbandcholesteric polarizer, in which a liquid-crystalline, cholestericallyordered layer comprising reactive chiral monomers and reactivenematogenic monomers of different reactivity is polymerized by exposureto radiation. The invention also relates to devices for carrying out themethod in accordance with the invention.

[0002] Broadband cholesteric polarizers and methods of manufacturingsame are known per se, for example, from Patent Publications EP-A404.939, EP-A 404.940 and WO 96/02016, which are all in the name of thecurrent Applicant. By means of cholesteric polarizers it is possible toconvert unpolarized light to circularly polarized light in asubstantially loss-free manner. Polarizers of this type comprise a thinlayer of a cholesterically (i.e. chirally nematically) ordered material.This material contains chiral, liquid-crystalline molecules having sucha structure that they order themselves more or less spontaneously into aspiral-shaped or helical structure. The pitch of this helix can beincreased by adding a quantity of a non-chiral, liquid-crystalline (i.e.nematogenic) material to the chiral, liquid-crystalline material. Theexact pitch is governed by the ratio between the quantities of chiraland non-chiral liquid-crystalline molecules as well as by their chemicalstructure.

[0003] If this material is provided in the form of a thin layer on asubstrate or between two substrates, said helical structure assumes suchan orientation that the axis of the helix extends transversely to thelayer. Such a layer is capable of reflecting a narrow band of lightwhose wavelength corresponds to the product of the pitch and therefractive index of the material and whose direction of polarizationcorresponds to the handedness of the helical structure. By virtue ofthis property, a cholesteric layer can very suitably be used in anoptical polarizer. It is noted that the expression “the refractiveindex” of a material is to be understood to mean in this context thegeometric mean (n_(e)+n_(o))/2 of the ordinary refractive index no andthe extraordinary refractive index n_(e) of this material.

[0004] Broadband cholesteric polarizers are distinguished from thecustomary cholesteric polarizers by the presence of a relatively broadreflection band. The bandwidth of the customary cholesteric polarizersis only approximately 40-50 nm. In the case of broadband polarizers,bandwidths of 100 nm, 150 nm, 200 nm and even more than 400 nm have beenachieved. It is noted that the band position of a cholesteric filter isdefined as the center of the wavelength range in which the reflectiontakes place. A width of a band is defined as the difference inwavelength between the long-wave and the short-wave edge positions ofthe band. The wavelength of an edge position is defined as thewavelength at which the intensity amounts to 50% of the maximumintensity.

[0005] EP 404.940 describes an elegant method of manufacturing abroadband cholesteric polarizer. In said method, use is made of amixture comprising reactive chiral monomers and reactive nematogenicmonomers, which exhibit a different reactivity. For the reactivemonomers use can be made of compounds containing a reactive group on thebasis of acrylates, epoxy compounds, vinylethers and thiolene systems,as described, inter alia, in U.S. Pat. No. 5,188,760. Monomerscontaining different reactive groups generally exhibit a differentreactivity. A difference in reactivity also occurs if one type ofmonomers contains one reactive group and the other type of monomerscontains two (identical) reactive groups.

[0006] A layer of this mixture is polymerized by means of (actinic)radiation, in particular UV radiation. In this process, the conditionsare selected in such a manner that during the polymerization operation aradiation profile of varied intensity is formed in the layer. As aresult, diffusion processes take place in the cholesteric layer duringpolymerization. This leads to a variation in the composition of thehelical structure, so that the pitch, viewed across the thickness of thelayer, varies within certain limits. As a result, this cholestericpolarizer exhibits a relatively broad reflection band.

[0007] It has been found that the method described in EP-A 404.940 canbe improved. The Applicant has experimentally established that smallfluctuations, for example in the radiation gradient or in the UVintensity, can strongly influence the diffusion processes of thereactive monomers. This may lead to relatively large differences in thebandwidth of the cholesteric polarizers manufactured by means of saidknown method. Therefore said known method should be improved, inparticular, with respect to the reproducible manufacture of polarizershaving a correct position of one of the two edges of the reflectionband. This applies, for example, to polarizers as described in thenon-prepublished European Patent Application having Application No.95203209.2 (PHN 15.556), in the name of the current Applicant.

[0008] It is an object of the invention to improve the known method. Theinvention more particularly aims at providing a method of manufacturingbroadband cholesteric polarizers of which the position of one of the twoedges of the reflection band can be adjusted in a very reproduciblemanner. The method in accordance with the invention should enable thesepolarizers to be mass-produced.

[0009] These and other objects of the invention are achieved by a methodof the type mentioned in the opening paragraph, which is characterizedin that the intensity of the radiation is increased substantially whenthe band reaches a desired edge position.

[0010] The invention is based on the experimentally gained insight thatthe intensity of the radiation used during polymerization plays animportant part in the manufacture of broadband polarizers. It has beendemonstrated that the eventually achieved bandwidth is governed to asubstantial degree by the radiation intensity used. If use is made of arelatively high UV intensity (typically 0.5 mW/cm² or higher), theeventually achieved bandwidth is found to be relatively small, and itdiffers hardly from that of the unpolymerized mixture. If a relativelylow radiation intensity (typically 0.05 mW/cm² or lower) is used, a muchbroader reflection band is obtained. Under these conditions, first, acolored, narrow reflection band is formed, which subsequently broadensinto an uncolored, broadband reflection band. A substantial increase ofthe intensity causes the bandwidth obtained at that instant to befrozen, as it were. It has been demonstrated that the increase inintensity should preferably be a factor of 10 or more to bring aboutsaid frozen state. Preferably, this factor is 20 or more. Under theseconditions, the cholesterically ordered layer becomes momentarypolymerized.

[0011] A preferred embodiment of the method in accordance with theinvention is characterized in that the attainment of the desired edgeposition of the band is determined by means of a monochromaticphotosensor, the wavelength used by said sensor corresponding to thewavelength of the desired edge position of the band. Such a photosensorcomprises a photodetector as well as a monochromatic light source. Alaser can very advantageously be used as the monochromatic light sourcein the sensor.

[0012] The sensor can be used in reflection. Said sensor is constructedin such a manner that the monochromatic light, which emanates from thelight source and which is used in the measuring operation, is directedto the layer to be polymerized. As long as the wavelength of the edgeposition of the reflection band is not equal to that of themonochromatic light, this light will pass through the layer to bepolymerized (transmission). As soon as the bandwidth assumes such avalue that said two wavelengths coincide, reflection of themonochromatic light occurs. A proper positioning of the layer, the lightsource and the detector causes this light to be reflected at thedetector. At this moment, the intensity of the polymerization radiationshould be increased. To this end, a second polymerization lamp having ahigher radiation intensity is activated or, preferably, a filtersituated in front of the polymerization lamp is removed. Instead of a(mechanically) movable filter, use can also advantageously be made of afilter whose transmission is adjustable.

[0013] Another preferred embodiment of the method in accordance with theinvention is characterized in that the desired edge position of the bandis determined by means of a transmission measurement. For this purpose,the sensor is constructed so that the light source and the detector aresituated on either side of the layer to be polymerized. In this case,the sensor is activated as soon as the detector detects a substantialreduction in intensity of the light emanating from the light source.Such an arrangement has the advantage that the exact alignment of thelayer to be polymerized does not affect the measuring results of thesensor.

[0014] Another suitable embodiment of the method in accordance with theinvention is characterized in that the liquid crystalline,cholesterically ordered layer is passed through an illumination tunnelwhich is provided with a number of compartments which comprise aradiation source as well as a light sensor by means of which theintensity of the radiation incident on the layer can be changed. Thisembodiment of the method in accordance with the invention enablesbroadband, cholesteric polarizers to be manufactured in a continuousprocess. This has a favorable effect on the cost price per unit area.

[0015] The invention also relates to a device for manufacturing abroadband cholesteric polarizer, which is characterized by a radiationcompartment comprising

[0016] a) means for positioning the polarizer to be manufactured,

[0017] b) a radiation source for the irradiation of the polarizer to bemanufactured and

[0018] c) a monochromatic photosensor comprising a photodetector as wellas a monochromatic light source.

[0019] This device in accordance with the invention enables broadbandpolarizers to be manufactured in batch processes. Preferably, thephotodetector and the monochromatic light source are positioned in theradiation compartment in such a manner that the sensor measures in thetransmission. To this end, the polarizer to be manufactured is situatedbetween the detector and the light source during operation of thedevice.

[0020] Another device in accordance with the invention, which is used tomanufacture a broadband cholesteric polarizer, is characterized in thatthe device comprises a number of radiation compartments, which areprovided with

[0021] a) means for passing a substrate through the radiationcompartments,

[0022] b) a radiation source for irradiating the substrate to be passedthrough said compartments, and

[0023] c) a monochromatic photosensor which comprises a photodetector aswell as a monochromatic light source.

[0024] This device in accordance with the invention enables broadbandpolarizers to be manufactured in a continuous process. Preferably, thephotodetector and the monochromatic light source are positioned in theradiation compartments in such a manner that the sensors measure intransmission. For this purpose, the—preferably elongated—substrate to bepassed through the radiation compartments is situated between thedetector and the light source during operation of the device.

[0025] To increase the radiation intensity, use can be made, forexample, of a second radiation source having a higher intensity. Thissource should be driven by the photosensor. A cheaper preferredembodiment of both devices in accordance with the invention is howevercharacterized in that the compartments comprise an optical filter whichcan be displaced by driving the sensor. Such a filter preferably passes10% or less of the radiation produced by the lamp. It is noted that anadjustable-transmission filter can be used instead of a displaceablefilter.

[0026] These and other aspects of the invention will be apparent fromand elucidated with reference to the embodiments described hereinafter.

[0027] In the drawings:

[0028]FIG. 1 schematically shows a broadband cholesteric polarizer whichcan be manufactured by means of the method in accordance with theinvention,

[0029]FIG. 2 shows the structure of a number of chemical compounds,

[0030]FIG. 3 schematically shows a first device in accordance with theinvention, which can be used to carry out the method according to theinvention,

[0031]FIG. 4 schematically shows a second device in accordance with theinvention, which can be used to carry out the method according to theinvention.

[0032] It is noted that the Figures are not drawn to scale.

[0033]FIG. 1 shows an embodiment of a broadband cholesteric polarizerwhich is manufactured by means of the method according to the invention.Said polarizer comprises two flat, transparent substrates 1 and 2, whichare made, for example, of glass and which are positioned substantiallyparallel to each other and at some distance from each other. The facingsurfaces of the substrates are provided with an orientation layer 3 and4, for example, of rubbed polyimide or sputtered SiO_(x). A spacer 5 isprovided at the edges of the substrates.

[0034] A layer 6 of a cholesterically ordered polymeric material issituated between said two substrates. The axis of the molecular helix ofthe cholesterically ordered material extends transversely to the layer.The molecular helix has a variable pitch which increases continuouslyfrom one surface of layer 6 to the other surface. This is schematicallyshown by means of two spiral-shaped structures 7. The thickness of layer6 typically ranges from 3 to 40 micrometers, preferably from 5 to 25micrometers.

[0035] The above-described embodiment of the cholesteric polarizer inaccordance with the invention was manufactured as follows. First, amixture containing reactive monomers was prepared. This mixturecontained 35 wt. % of the chiral monomer A and 65 wt. % of thenematogenic monomer B. Monomer A contains one reactive group permolecule and monomer B contains two reactive groups per molecule. Inthis case, acrylate groups were used. The exact structural formulas ofthe monomers A and B are shown in FIG. 2. The difference in reactivitybetween both monomers can be attributed to the different number ofreactive groups per molecule. Subsequently, 2 wt. % of thephotoinitiator Irgacure 651 (Ciba Geigy; structural formula C) and 0.001wt. % p-methoxyphenol (stabilizer; structural formula D) as well as 1wt. % of a dye (structural formula E) were added to this mixture. Thisdye exhibits an absorption maximum around 334 nm and an extinctioncoefficient of 31524 l/mol.cm. This dye enables the intensity gradientof the radiation used to be set more easily. This is described ingreater detail in EP 606.940.

[0036] The mixture thus prepared was subsequently provided between twotransparent substrates. These substrates were provided with a layer of arubbed polyimide. These layers serve to improve the alignment of themolecular helix, which develops spontaneously in the cholestericmixture. It is noted that, in said method according to the invention, itis not absolutely necessary to use substrates which are provided withorientation layers. In the manufacture of (very) thin optically activelayers, generally spontaneous orientation of the chiral and nematogrenicgroups takes place. However, the presence of orientation layers duringpolymerization does lead to an improved orientation of the opticallyactive layer, so that the optical properties of the polarizers areimproved substantially.

[0037] The polymerization of the cholesteric layer will be explained ingreater detail by means of the schematic, sectional view of theinventive device shown in FIG. 3. This device comprises a radiationcompartment 11. This is provided with means for positioning thebroadband cholesteric polarizer to be manufactured. In the device shown,these means are constructed as support bodies 12. In the present case,the polarizer comprises two substrates 13 and a cholesterically orderedlayer 14 situated between said substrates. It is noted that the methodand device in accordance with the invention can also be used tomanufacture polarizers in which only one substrate is used.

[0038] Compartment 11 further comprises a radiation source in the formof an UV lamp. The power of the lamp and the distance between the lampand the polarizer to be irradiated are selected in such a manner thatthe average illumination intensity to which the cholesteric layer isexposed during operation of the device is approximately 0.9 mW/cm². Aneutral-density filter 16 whose position or transmission is adjustableis arranged between the UV lamp and the polarizer. When the filteroperates at maximum capacity, the radiation originating from the UV lampis filtered in such a manner that the average illumination intensity onthe polarizer is only approximately 0.03 mW/cm².

[0039] Compartment 11 also comprises a photosensor which consists of amonochromatic light source 17 in the form of a laser and of aphotodetector 18. The wavelength of the laser is selected to be suchthat it is equal to the edge position of the broadband polarizer to bemanufactured. The sensor measures in transmission, so that the detectorand the light source are situated on opposite sides of the polarizerwhich is the subject of measurements. The sensor is coupled to thefilter 16. As soon as the transmission of the polarizer decreasessubstantially (50% or more) during illumination, the sensor supplies asignal which causes the neutral density filter 16 to be activated.Depending on the type of filter, this is either removed from itsposition between the radiation source 15 and the polarizer or thetransmission of the filter is maximized. By virtue thereof, theradiation intensity on the polarizer increases by a factor of 30, whichresults in the instantaneous, complete polymerization of the cholestericlayer. As a result, the bandwidth as well as the exact position of oneof the band edges is defined.

[0040]FIG. 4 is a schematic, sectional view of a device in accordancewith the invention for mass-producing broadband cholesteric polarizers.This device comprises a temperature-controlled radiation tunnel 21 whichaccommodates a number of radiation compartments 22. Said radiationtunnel is provided with means 23, for example in the form of drivablerollers, enabling an elongated, flexible substrate 24 to be continuouslyfed through. This substrate may consist, for example, of a thin,transparent foil carrying a cholesterically ordered layer of a mixtureof reactive, liquid-crystalline monomers to be polymerized.

[0041] The compartments are provided with a radiation source 25, forexample in the form of an UV lamp which is used to irradiate thesubstrate while it is being fed through the radiation tunnel. A numberof said compartments are provided with a photodetector 26 and with amonochromatic light source in the form of a single laser 27. In thepresent case, the partially transmissive mirrors 28 divide the laserbeam of laser 27 into a number of deflecting sub-beams which areincident on the photodetector via the substrate.

[0042] The method in accordance with the invention can be appliedcontinuously in the device in accordance with the invention in thefollowing manner. A substrate which is transparent to the laser lightused is provided with a cholesterically ordered layer of aliquid-crystalline material. This substrate is passed through theradiation tunnel by drive means. In this process, the substrate is fedpast a number of radiation compartments in which it is exposed to an UVlamp of a relatively low intensity (0.05 mW/cm² or less).

[0043] At a given moment, the reflection band will have reached such awidth, as a result of the polymerization process, that the wavelength ofone of the edge positions is equal to that of the laser used. At thatmoment, the intensity measured by the photodetector is reducedsubstantially. A signal is then given which causes the radiationintensity to be increased substantially, for example, by a factor of 10or more. As a result, instantaneous, complete polymerization of theliquid-crystalline material takes place, so that the measured edgeposition and bandwidth become frozen, as it were. The egressingsubstrate with the polymerized, broadband, cholesterically ordered layercan be processed further, in a manner which is well-known to thoseskilled in the art, to form a broadband cholesteric polarizer. To thisend, the substrate is cut to the proper dimensions and, if necessary,provided with a quarter lambda foil if the transmitted light should becircularly polarized. If necessary, the contrast can be increased byproviding the filter with a dichroic polarization foil.

[0044] The method in accordance with the invention enables broadbandcholesteric polarizers having an accurately adjusted edge position to bemanufactured. By virtue thereof, the viewing-angle dependence of adisplay provided with such a polarizer can be reduced. By means of thedevices in accordance with the invention, the polarizers can be producedin batch processes or continuous processes.

1. A method of manufacturing a broadband cholesteric polarizer, in which a liquid-crystalline, cholesterically ordered layer comprising reactive chiral monomers and reactive nematogenic monomers of different reactivity is polymerized by exposure to radiation, characterized in that the intensity of the radiation is increased substantially when the band reaches a desired edge position.
 2. A method as claimed in claim 1 , characterized in that the increase of the radiation intensity amounts to a factor of 10 or more.
 3. A method as claimed in claim 1 or 2 , characterized in that the attainment of the desired edge position of the band is determined by means of a monochromatic photosensor, the wavelength used by said sensor corresponding to the wavelength of the desired edge position of the band.
 4. A method as claimed in claim 3 , characterized in that the desired edge position of the band is determined by means of a transmission measurement.
 5. A method as claimed in any one of the preceding claims, characterized in that the liquid crystalline, cholesterically ordered layer is passed through an illumination tunnel which is provided with a number of compartments which comprise a radiation source as well as a light sensor by means of which the intensity of the radiation incident on the layer can be changed.
 6. A device for manufacturing a broadband cholesteric polarizer, which is characterized in that the device comprises a radiation compartment having a) means for positioning the polarizer to be manufactured, b) a radiation source for the irradiation of the polarizer to be manufactured and c) a monochromatic photosensor comprising a photodetector as well as a monochromatic light source.
 7. A device for manufacturing a broadband cholesteric polarizer, which is characterized in that the device comprises a number of radiation compartments, which are provided with a) means for passing a substrate through the radiation compartments, b) a radiation source for irradiating the substrate to be passed through said compartments, and c) a monochromatic photosensor which comprises a photodetector as well as a monochromatic light source.
 8. A device as claimed in claim 6 or 7 , characterized in that the compartments comprise an optical filter which can be displaced by driving the sensor.
 9. A device as claimed in claim 6 or 7 , characterized in that the compartments comprise an optical filter whose transmission can be adjusted. 